When populating ExportMap, only load file if it's not ignored

CHANGELOG.md

@@ -13,6 +13,7 @@ This change log adheres to standards from [Keep a CHANGELOG](http://keepachangel
 
 ### Fixed
 - `default`: make error message less confusing ([#1470], thanks [@golopot])
+- Improve performance of `ExportMap.for` by only loading paths when necessary. ([#1519], thanks [@brendo])
 - Support export of a merged TypeScript namespace declaration ([#1495], thanks [@benmunro])
 - [`import/order`]: fix autofix to not move imports across fn calls ([#1253], thanks [@tihonove])
 
@@ -609,6 +610,7 @@ for info on changes for earlier releases.
 
 [`memo-parser`]: ./memo-parser/README.md
 
+[#1519]: https://github.com/benmosher/eslint-plugin-import/pull/1519
 [#1495]: https://github.com/benmosher/eslint-plugin-import/pull/1495
 [#1472]: https://github.com/benmosher/eslint-plugin-import/pull/1472
 [#1470]: https://github.com/benmosher/eslint-plugin-import/pull/1470
@@ -996,3 +998,4 @@ for info on changes for earlier releases.
 [@TrevorBurnham]: https://github.com/TrevorBurnham
 [@benmunro]: https://github.com/benmunro
 [@tihonove]: https://github.com/tihonove
+[@brendo]: https://github.com/brendo

src/ExportMap.js

@@ -310,11 +310,18 @@ ExportMap.for = function (context) {
     return null
   }
 
+  // check for and cache ignore
+  if (isIgnored(path, context)) {
+    log('ignored path due to ignore settings:', path)
+    exportCache.set(cacheKey, null)
+    return null
+  }
+
   const content = fs.readFileSync(path, { encoding: 'utf8' })
 
-  // check for and cache ignore
-  if (isIgnored(path, context) || !unambiguous.test(content)) {
-    log('ignored path due to unambiguous regex or ignore settings:', path)
+  // check for and cache unambigious modules
+  if (!unambiguous.test(content)) {
+    log('ignored path due to unambiguous regex:', path)
     exportCache.set(cacheKey, null)
     return null
   }